drm/exynos: fix checks for valid mixer window

Valid values for mixer window are from 0 to MIXER_WIN_NR-1 inclusive.
Arrays in structures (e.g. mixer_context.win_data) have size of
MIXER_WIN_NR so checks for wrong mixer window must be greater-equal.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Hyunhee Kim <hyunhee.kim@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>